A “synthesis gas production method” as described in PTL 1 is known as a related-art synthesis gas production method. This production method produces a synthesis gas (a mixed gas of carbon monoxide and hydrogen) while minimizing energy necessary for a reaction by optimally combining three reactions of (1) a dry reforming reaction of reacting a hydrocarbon compound mainly rich in methane with carbon dioxide, (2) a steam reforming reaction of reacting the hydrocarbon compound with steam, and (3) an autothermal reaction of reacting the hydrocarbon compound with oxygen.
The reactions (1) to (3) above are specifically reactions shown below. The reactions (1) and (2) are an endothermic reaction, and the reaction (3) is an exothermic reaction.CH4+CO2→2CO+H2+284 kJ/mol  (1)CH4+H2O→CO+3H2+206 kL/mol  (2)CH4+½O2→CO+2H2-35.6 kJ/mol  (3)
It is preferred as raw materials for a chemical product that carbon monoxide and hydrogen in a synthesis gas are present in a proportion of 1:2 (molar ratio). PTL 1 describes an optimized production method by combining the reactions (1) to (3) described above such that a synthesis gas ratio is optimized and energy necessary for a reaction is minimized and by supplementing insufficient energy by utilization of natural energy such as sunlight heat.
In “a method for producing hydrogen and carbon monoxide using methane as a raw material” described in PTL 2, a method for continuously producing hydrogen or carbon monoxide by contacting a natural gas containing methane with a metal oxide containing a rare earth at from 200 to 1,000° C. to react with lattice oxygen, thereby reducing the metal oxide, and then reacting the reaction medium reduced with steam or carbon dioxide is disclosed.
The reaction in PTL 2 is represented by the following formula (4). In this reaction formula, rare earth oxide is expressed as MnOm (M is a rare earth element).
X is a stoichiometric coefficient.X.CH4+MnOm→2X.H2+X.CO+MnOm-x  (4)
The case that cerium is used as rare earth is represented by the following formula (5).X.CH4+CeO2→2X.H2+X.CO+CeO2-x  (5)
In the reaction of the formula (5), carbon in methane bonds to oxygen in cerium oxide to generate CO, and additionally hydrogen remained in methane is generated. As a result, CO and H2 that are synthesis gases are produced in a certain ratio. When cerium oxide whose oxygen has been reduced is reacted with CO2 or H2O, the cerium oxide of the initial state can be refreshed as shown in the formulae (6) and (7), and this simultaneously makes it possible to produce CO or H2.CeO2-x+X.CO2→CeO2+X.CO  (6)CeO2-x+X.H2O→CeO2+X.H2  (7)